The present invention relates generally to transducers for converting audio signals to audible vibrations, and more particularly to hearing devices with improved energy efficiency, sound fidelity, and inconspicuousness.
For the sake of a better understanding by the reader of the improvements provided by the present invention, it is useful to offer a brief description of the human ear canal anatomy and physiology. The external acoustic meatus (ear canal) is generally narrow and tortuous as shown in the coronal view in FIG. 1. The ear canal 10 is approximately 23 to 29 millimeters (mm) long from the canal aperture 17 to the tympanic membrane (eardrum) 18. The lateral part of ear canal 10 is a relatively soft region 11 because of underlying cartilaginous tissue, and moves in response to motions of the subject's jaw which occur during talking, yawning, eating, and so forth. Cerumen (earwax, not shown) production and hair growth 12 occur primarily in this cartilaginous region. The medial part of the canal is a bony region 13 which is rigid because of underlying bony tissue, and lies proximal to the tympanic membrane 18. The skin 14 in bony region 13 is thin relative to skin 16 in cartilaginous region 11, and is sensitive to touch or pressure. A characteristic bend 15 that roughly separates cartilaginous region 11 and bony region 13 has a magnitude which varies significantly among individuals. The cross-sectional shape (not shown) of ear canal 10 is generally oval with a long/short (vertical/horizontal) axis ratio ranging from 1:1 to 3:1. The diameter ranges from as little as 3 mm (along the horizontal axis of the bony region in small canals) to as much as 16 mm (along the vertical axis of the cartilaginous region in large canals).
Physiological debris including sweat, cerumen and oils produced by the various glands underneath the skin, are often present in the ear canal.
Ear canal 10 terminates at and is separated from the middle ear cavity 21 by the tympanic membrane 18, which is generally oval (FIG. 2) and conical (FIG. 1), with a characteristic dip at the umbo area 20 (FIGS. 1 and 2). The tympanic membrane weighs approximately 14 milligrams (mg) and is connected to the handle of the malleus ossicle 19, which itself has a weight in a range from about 22 to about 32 mg. The malleus ossicle is connected to other ossicles (incus 22 and stapes 23) and ligaments (not shown) within the middle ear cavity. Tympanic membrane 18 and associated middle ear ossicles 19, 22 and 23 are extremely sensitive to pressure waves which are imperceptible by even the most delicate receptors of skin.
Hearing loss affects a substantial percentage of the population, and is of several types. The loss occurs naturally with aging, beginning with the higher frequencies (4000 Hz and above) and increasingly spreads to lower frequencies. Conductive losses attributable to damage or disease of the tympanic membrane and associated ossicies also effect the hearing in the lower frequency range. It is customary, of course, to fit individuals who suffer from hearing loss with hearing aid devices, which are of many different types.
In general, conventional hearing devices rely primarily on air-conduction transducers to produce pressure waves which are transmitted to the tympanic membrane through the air between the transducer and the tympanic membrane. These transducers, also referred to as receivers or speakers, are used in various audio devices including hearing aids, telephones, radios and televisions. For such hearing devices, the efficiency of air-conduction is generally inversely proportional to the distance or residual volume between the receiver and tympanic membrane. The closer the receiver is to the tympanic membrane, the smaller the air mass between them, and thus the lower the energy required to vibrate the tympanic membrane.
Significant advances have been made in hearing aid receiver design during the past two decades, in energy efficiency, size and acoustic distortion reduction. These advances have led to a new class of miniature hearing devices that fit deeply in the ear canal, with receivers close to the tympanic membrane. Such devices are largely inconspicuous, and thereby tend to alleviate the social stigma and vanity concerns associated with wearing a visible hearing aid, which are considered the primary obstacles to use among the hearing impaired population. Nevertheless, a number of fundamental limitations remain in hearing devices that utilize air-conduction based technology, including problems of (1) frequent device handling, (2) acoustic feedback, (3) ear canal occlusion, and (4) low sound fidelity.
The problem of frequent device handling relates to the need, with conventional hearing devices, for frequent insertion and removal from the ear canal. Conventional hearing aids are typically removed daily to relieve the ear canal from device pressure and to aerate the ear canal and the tympanic membrane. The requirement of frequent handling, particularly with miniature hearing devices, poses a serious challenge especially to individuals who suffer physical impairment beyond hearing loss because of age or disorders, such as arthritis, tremors, or other neurologic problems.
Device removal is also required for battery replacement. For miniature canal devices (the term "canal devices" refers to miniature hearing devices that are primarily fitted in the ear canal, and includes In-The-Canal (ITC) devices and Completely-In-the-Canal (CIC) devices), typical battery lifetimes range from one week to four weeks. The need for frequent battery replacement is attributable in large part to the magnitude of energy consumption by conventional air-conduction receivers. State-of-the-art receivers consume electrical power in a range from 250 to 1000 microwatts (.mu.W) to produce acoustic signals audible by the typical hearing-impaired individual (discussed below in the section regarding experiment B). Even with the most efficient currently available battery technology and dramatic reduction in power consumption of all other components of the hearing device, the receiver power consumption alone will lead to complete battery depletion within two to four weeks, depending on the amplification level (hearing loss). Battery type and size are limited because of typical ear canal size and shape constraints discussed above. For example, if a type 10A Zinc-Air battery (which represents the state of the art in miniature hearing aid batteries, having energy capacity of about 60 milliampere-hours (mA-Hr)) is employed with a conventional air-conduction receiver which consumes about 250 microamps (.mu.A), the battery life will be only about 17 days, assuming a typical device use of 14 hours per day. Actual battery lifetime is shorter because of the additional power demands by other components of the hearing aid (not considered in the above calculation).
The problem of acoustic feedback occurs when a portion of the sound output, typically from a receiver (speaker), leaks to the input of the hearing system such as a microphone of a hearing aid. Such leakage often causes a sustained oscillation which is manifested by "whistling" or "squealing". Acoustic feedback, which is not only annoying to hearing aid users but also interferes with their speech communication, is a common occurrence in conventional hearing aids since the output of the device (acoustic) is in the same form of energy as the input of the device (also acoustic). Feedback is typically alleviated by occluding (sealing) the ear canal tightly with the hearing device. An additional sealing element may also be used to alleviate feedback as described in U.S. Pat. No. 5,682,020 to Oliviera and U.S. Pat. No. 5,654,530 to Sauer. Whichever acoustic sealing method is used, ear canal occlusion causes an array of side effects.
Occlusion related problems include discomfort, irritation and even pain; moisture build-up in the occluded ear canal; cerumen impaction; and occlusion effect. Discomfort, irritation and pain may occur from canal abrasion caused by frequent insertion and removal of a tightly fitted hearing device. The conventional hearing aid housing is typically made of custom shaped plastic material (e.g., acrylic) which easily causes pressure to and abrasion of the ear canal. A rigid enclosure is necessary to protect components within the hearing device during the daily handling routine. As observed by M. Chasin in CIC Handbook, Singular Publishing (1997), canal discomfort and abrasion result in frequent return of hearing devices to the manufacturer, seeking improved custom fit and comfort. Chasin further notes that long term effects of the hearing aid include atrophy of the skin and a gradual remodeling of the bony canal, with chronic pressure on the skin lining the ear canal which causes thinning of that layer and possible loss of skin appendages.
Moisture build-up in the occluded ear canal causes damage to the ear canal and the hearing device within. Chasin (ibid) further observes that humidity increases rapidly in the occluded portion of the canal, and is aggravated by hot and humid weather, exercise, and a tympanic membrane perforation; deep canal water saturation is higher than the ambient atmospheric humidity even with venting; and, since normally present bacteria thrive in an environment of high humidity and altered pH, the ear is now prone to infection. To reduce these damaging effects of canal moisture, it is often recommended that hearing devices be removed daily.
Chasin also states that cerumen impaction (i.e., blockage of the ear canal by ear wax) may occur when ear wax is pushed deeper in the ear canal by the inserted hearing device. Cerumen can also build up on the receiver of the hearing device, thereby causing frequent malfunction, and indeed, as Oliveira et al have observed (in The Wax Problem: Two New Approaches, The Hearing Journal, Vol. 46, No. 8), cerumen contamination is probably the most common factor leading to hearing aid damage and repair.
The occlusion effect is a common acoustic problem caused by the occluding hearing device, manifested by the perception of a person's own-voice ("self-voice") being loud and unnatural compared to that with the open ear canal. This phenomenon is sometimes referred to as the "barrel effect" since it resembles the experience of talking into a barrel. Referring to FIG. 3, the occlusion effect is generally related to self-voice 60 resonating within the ear canal. In an ear canal occluded by a hearing device 70, a large portion of the self-voice 60, originating from the larynx (voice-box) and conducted upward by various body structures, is directed at tympanic membrane 18, as shown by arrow 61. Even when a vent 71 is used, allowing a portion of self-voice 60 to escape as shown by arrows 62 and 62', the residual "trapped" sound energy 61 is perceived by the individual wearing the device as being loud or unnatural.
In the open (non-occluded) ear canal, shown in FIG. 4, a relatively larger amount of self-voice 60 is allowed to escape (arrow 63). The residual sound (arrow 64) directed at the tympanic membrane 18 is relatively smaller and is perceived by the wearer as natural self-voice. For hearing aid users, the occlusion effect is inversely proportional to the residual volume of air between the occluding hearing device and the tympanic membrane. Therefore, the occlusion effect is considerably alleviated by deep insertion of the device into the ear canal.
Low or inadequate sound fidelity is often experienced with air-conduction receivers (speakers), particularly in hearing aid applications. The acoustic response of an air-conduction speaker is characteristically limited to a particular range of frequencies. In the case of a high fidelity speaker system, for example, a limited frequency range exists but the system is designed using multiple speakers (e.g., woofers, tweeters, etc.) to achieve a broader frequency response. Unfortunately, space limitations in the ear canal do not allow for multiple receivers, and receivers which are used in canal devices are generally limited to a frequency range between 200 and 5000 Hz.
The limitations of conventional air-conduction hearing devices cited above are highly interrelated. For example, as Chasin (id.) observes, when a hearing aid is worn in the ear canal, movements in the cartilaginous region may cause slit leaks that result in feedback, discomfort, occlusion effect, and ejection of the device from the ear. Often, the relationship between the limitations is adverse. For example, occluding the ear canal tightly is desirable to prevent oscillatory feedback, but is to be avoided if one is seeking to prevent or diminish the various side effects of occlusion. The use of a vent 71 (FIG. 3) to alleviate occlusion effect provides an opportunistic pathway (74 and 74') for acoustic leakage between the air-conduction receiver 73 and the microphone 72, which tends to cause feedback. For this reason, the vent 71 in CIC devices is typically limited to a diameter in the range from 0.6 to 0.8 mm (see Chasin, id.).
Considering the state of the art in alternative hearing device technology, hearing devices employing transducers that are not based on air-conduction are well known in the art. The rational is that when no acoustic output is present in such devices, oscillatory feedback is usually reduced and in most cases eliminated. Distortion and frequency response characteristics are also potentially improved.
For example, vibratory middle ear implants attempt to circumvent some of the above-cited limitations by vibrating directly any of the ossicular (middle ear bones) or cochlear structures. Vibratory transducers and hearing devices for middle ear implant are disclosed in numerous patents, e.g., U.S. Pat. No. 3,594,514 to Wingrove, U.S. Pat. No. 3,764,748 to Branch, U.S. Pat. No. 3,870,832 to Fredrickson, U.S. Pat. No. 3,882,285 to Nunley et al, U.S. Pat. No. 5,015,224 to Maniglia, U.S. Pat. No. 5,282,858 to Bisch et al, U.S. Pat. No. 5,531,787 to Leisinski, U.S. Pat. Nos. 5,554,096 and 5,456,654 to Ball, and U.S. Pat. No. 5,730,699 to Theodore et al. The transducer technology employed includes piezoelectric and electromagnetic elements which provide electrical output via an electrical wire connection to the transducer. Disadvantages of middle ear implants include the cost and risk involved in the surgical procedure, and the additional surgery that may be required to repair device malfunctions or to replace an implanted battery.
Several other hearing systems that are less invasive have been proposed and are known in the art. Magnetic transducers which are surgically implanted or surgically attached to the tympanic membrane are disclosed in a number of patents, e.g., U.S. Pat. Nos. 4,840,178 and 5,220,918 to Heide et al, U.S. Pat. No. 4,817,607 to Tatge et al, U.S. Pat. Nos. 4,606,329, 4,776,322 and 5,015,225 to Hough et al, U.S. Pat. No. 4,957,478 to Maniglia, U.S. Pat. No. 5,163,957 to Sade et al, and U.S. Pat. No. 5,338,287 to Miller et al. These transducers typically employ high energy product magnets which vibrate in response to a radiant electromagnetic signal, representative of acoustic signals. The electromagnetic signal is typically radiated by a coil positioned in the external ear canal (e.g., 44 of FIG. 1 in the Manigila '478 patent, and 28 of FIG. 1 in the Tatge '607 patent). Similarly, a primary disadvantage of this type of device is the cost and risk of surgery performed on the delicate vibratory structures of the ear.
Among others of the less invasive approaches are those proposed in U.S. Pat. No. 5,259,032 to Perkins et al, and U.S. Pat. No. 5,425,104 to Shennib. In each of these disclosures, a magnet transducer is attached non-surgically to the exterior side of the tympanic membrane, and the transducer receives radiant electromagnetic signals from a device in the ear canal (FIG. 4 of the Perkins et al '032 patent), or from an externally positioned coil (FIGS. 1A and 1B of the Shennib '104 patent).
A major disadvantage with all of the above electromagnetic hearing systems is the inefficiency associated with transducing radiant electromagnetic energy into magnet vibrations, attributable to the relatively small portion of radiant electromagnetic energy produced by the coil that reaches the magnet. As is known in the art of electromagnetics, the efficiency of such coupling is inversely proportional to the distance between the driving coil and the magnet transducer. For example, a large externally positioned coil consumes about 1 ampere peak to produce roughly the same perceived sound pressure level as a small coil within the ear canal consuming only 5 mA peak (see the Shennib '104 patent). However, even for devices with small coils that are positioned deep in the ear canal proximal to the tympanic membrane, the power consumption is prohibitive for practical applications. This and other limitations of such devices render the various modes of radiant electromagnetic transconduction impractical for hearing aid applications.
A potentially more energy efficient transducer and hearing system is disclosed in U.S. Pat. No. 5,624,376 to Ball et al. In a non-invasive embodiment of the transducer disclosed in FIG. 19a of the Ball et al '376 patent, a floating mass transducer 100 is attached non-surgically to the exterior side of the tympanic membrane via an attachment membrane 502. The transducer 100 may be directly connected (not shown, but disclosed at col. 16, line 62) to a hearing device 506 via electrical wires 24. The "floating mass transducer" (FIG. 3), incorporates a magnet 42 (floating mass) and a coil 14 within a housing 10. The transducer 100 is free to vibrate within the housing 10 in response to the electrical signal via wires 24. The inertial forces of the vibrating magnet cause the housing to vibrate and subsequently vibrate the attached tympanic membrane and ossicles. According to the Ball et al '376 patent, vibration forces are maximized by optimizing the mass of the magnet assembly relative to the combined mass of coil and housing, and the energy product of the permanent magnet.
Since the transducer receives electrical energy directly from the hearing device via the electrical wire, energy loss is reduced and the device is potentially more energy efficient than air-conduction or radiant electromagnetic hearing systems. But a major disadvantage of the floating mass transducer is the weight of the transducer assembly. In a transducer example described at col. 22 of the Ball et al '376 patent, a NdFeB magnet of 2 mm in diameter and 1 mm length was employed, which has a calculated weight (magnet alone, from the volume and density of NdFeB 7.4 gm/cm.sup.3) of approximately 23 mg, which well exceeds the typical weight of the tympanic membrane (14 mg).
Another alternative to air-conduction hearing devices is disclosed in U.S. Pat. Nos. 4,628,907 and 4,756,312 to Epley. The Epley '907 patent describes a canal hearing device with an electromechanical transducer part directly contacting the tympanic membrane (FIG. 1), the contact element 38 being secured to the tympanic membrane by clip means for attachment to malleus bone (claim 1). The devices are not only invasive as disclosed, but also pose a considerable risk to the delicate structures of the tympanic membrane from inadvertent movement of the hearing device, which may occur, for example, simply by normal jaw motion.
Many of these prior art devices are occlusive to the ear canal which render them impractical for long term use. As used in the present application, long term use means continuous placement and operation of a hearing device within the ear canal for at least one month.
A key goal of the present invention is to provide a highly energy efficient sound conduction means by vibrating directly the tympanic membrane without resorting to a transducer placed directly on the tympanic membrane.
Other goals of the present invention include the design of an inconspicuous and non-occlusive canal hearing aid for long term use.